1. Field of the Invention
Embodiments of the present invention relate generally to photovoltaic devices and, more specifically, to a method of controlling the process parameters during a scribing process.
2. Description of the Related Art
As photovoltaic industry matures, there is a growing need for increased process control in the production of solar cells. Tighter process control can improve yield and allow scaling of current fabrication processes to larger substrates. To cost effectively and efficiently form multiple solar cell devices (referred to herein as “solar modules”), various solar cell formation process parameters throughout the fabrication process need to be effectively controlled. One set of such parameters is related to a scribing process, such as a laser scribe process, performed at various stages of solar module fabrication process.
The three major parameters used to control a laser scribe process are laser scan speed, frequency of the laser, and output current of the laser. The laser scan speed is dictated by manufacturing throughput requirements and is typically set at some constant value. The frequency and the output current of the laser govern the actual power produced by the laser. When performing laser scribing in various stages of the solar module fabrication, if the laser power is too low, the laser does not generate enough heat to cut through, or ablate, a respective layer. If, however, the laser power is too high, the laser cuts not only through the respective layer but may also cut or damage the layer(s) or substrate disposed underneath. Therefore, tuning the laser scribe parameters to achieve the laser power necessary to make a clean cut through a specific layer of the solar module is important.
One approach to optimizing the laser scribe parameters includes making a laser scribe to produce a trench in a specific layer of a solar module, taking the unfinished solar module out of the laser scribing chamber, using a profilometer to determine the dimensions of the trench and the roughness of the bottom of the trench, adjusting the laser scribe parameters to improve subsequent laser scribes, and then placing the solar module back into the laser scribing chamber or discarding the processed substrate. Drawbacks of using such an off-line measurement technique can include an increased laser scribe cycle time and cost of producing a solar cell.
Another approach to optimizing the scribing process parameters includes performing direct measurements (e.g., resistance measurements) using standard test equipment on a finished solar module. However, measurement limitations hinder the ability to obtain granular visibility into the laser scribe process and perform efficient process control. In addition, since the information provided by such measurements is often limited to the performance of finished solar modules, thus determining the root cause of poorly performing solar cell modules and fully understanding and characterizing a formed solar module may be challenging.
As the foregoing illustrates, what is needed in the art is a technique for monitoring and tuning a scribing process that avoids the drawbacks of the prior art approaches.